1. Field of Invention
The present invention relates generally to ultra wideband (UWB) communication systems. More specifically, it relates to the transmission, reception and detection of impulse radio signals in a transmitted reference (TR) system.
2. Background of Related Art
Ultra Wideband (UWB) technology has been proposed as a promising physical layer candidate for the short-range high-data-rate indoor wireless communications. By the low-duty-cycle transmission of short pulses with a pulse width on the order of sub-nanoseconds, it allows to minimize the power consumption at the radio units and provides fine multipath resolvability. The latter is especially desirable for indoor wireless communications where the channel exhibits dense multipath characteristic.
However, the fine multipath resolvability also provides challenges to the design of a low-complexity and low-cost UWB receiver. To fully collect the signal energy spreaded over all the multipath components (MPCs), a commonly-used Rake receiver has to implement tens or even hundreds of correlation branches. On the other hand, using only a subset of MPCs, notably selective or partial Rake, sacrifices the signal energy for a moderate-complexity implementation of the receiver. In addition, the Rake receiver requires channel estimation to combine the MPCs, which further increases the receiver complexity as the number of MPCs grows.
For the above reasons, much attention has been directed to the use of transmitted reference (TR) system. One such UWB-TR system is disclosed in the U.S. patent “Ultra-Wideband Communications System”, numbered as U.S. Pat. No. 6,810,087 B2. As disclosed therein, an unmodulated reference pulse is transmitted before each data-modulated pulse to provide an immediate channel estimation for detecting the data bit modulated on the data pulse. In order for the two pulses to experience the same channel condition, the time separation between the two pulses should be less than the channel coherence time. The TR receiver first correlates the reference pulse with its associated data pulse. In the output of the correlator, each MPC results in a peak and all peaks share the same polarity determined by the modulated data bit. By integrating the correlator output for a certain time interval, the signal energy spreaded over the MPCs is coherently added and then used to detect the modulated data bit. Compared to the Rake receiver, the TR scheme does not require channel estimation and it is suitable in dense multipath environments with manageable receiver complexity. However, these benefits are achieved at the expense of power inefficiency by sending the reference pulse and noise enhancement due to the noisy reference, where the latter results in large noise-times-noise terms which degrade the detection performance severely at the low or medium signal-to-noise ratio (SNR) range. In addition, to avoid the inter-pulse interference (IPI) between the reference and data pulses, the time separation between them has to be at least equal to the channel delay spread, which indicates a loss in the data rate.
In order to improve the detection performance of the TR system, most approaches in the prior art focus on getting a better template signal through noise averaging prior to the correlation. Notable among them is a hybrid matched filter correlation receiver, disclosed in the U.S. patent application Publication Number US 2005/0013390 A1, titled as “Hybrid UWB receiver with matched filters and pulse correlator”. As disclosed, by applying a symbol-rate matched filter before the cross correlator, the receiver not only achieves noise-averaging but also allows the subsequent digital processing to operate at the symbol rate rather than the frame rate, making it superior to prior TR schemes. However, this scheme, referred to as conventional TR hereafter, and other prior art TR schemes, are susceptible to the IPI occurred when the reference and data pulses are separated by a distance less than the channel delay spread. As a result, the achievable data rate is much limited. To deal with this, there is one proposal based on maximum-likelihood template estimation in the presence of IPI. However, it is too expensive to implement.
Therefore, there exists a need for a low-complexity UWB-TR system that is not subject to the IPI, and more suitable for high-data-rate communications.